Recently, according to the rapid progress of miniaturization, weight reduction and high-performance of electronics, electronic equipments and communication devices, improvement of performance of a secondary battery, which is used as a power supply of the product, is greatly required. A secondary battery satisfying these requirements may be a lithium secondary battery, and the lithium secondary battery may be largely classified into a lithium sulfur battery using a sulfur-based material as a positive electrode active material and a lithium ion battery using a lithiated transition metal oxide as a positive electrode active material.
The positive electrode active material is a material playing the most importance role to the battery performance and safety of the lithium secondary battery, and it may be a chalcogenide compound. For example, composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1) and LiMnO2 are being studied. This positive electrode active material is mixed to a conducting material such as carbon black, a binder and a solvent to prepare a positive electrode active material slurry composition, and then the composition is coated on a thin metal plate such as aluminum foil to be used as a positive electrode of a lithium ion secondary battery.
Among the positive electrode active materials, the Mn-based positive electrode active material such as LiMn2O4 and LiMnO2 is attractive material because it is easy to synthesize, cheap and environment-friendly, but it has a shortcoming of small capacity. The Co-based positive electrode active material such as LiCoO2 shows good electric conductivity, high battery voltage and excellent electrode characteristics, but it has a shortcoming of expensive price.
In order to overcome the shortcomings of the Co-based positive electrode active material such as LiCoO2, in recent, researches for developing high capacity batteries are in progress actively. Further, compared with a mobile field such as mobile phone, personal computer, a power tool field such as power tool and a power-driven field such as electric motor motorcycle and electric motor assistance bicycle have large load fluctuation to a power supply, and also used for a long time continuously. Accordingly, high output and high capacity power supplies are demanded.
In general, in a battery, high capacity and high output are contradictory performances, and are difficult to be compatible. For example, a non-aqueous electrolyte battery such as a lithium ion secondary battery as a representative high capacity-type secondary battery can be continuously discharged for a long time due to low load of about 0.2 C. Accordingly, it is mainly used as a power supply for a mobile field and the like, but current density per electrode area at low load is only 0.01 A/cm2. For this reason, the high capacity-type non-aqueous electrolyte battery is not suitable for a power supply for high load used in a power-driven field requiring discharge at high current where current density is 0.1 A/cm2 and more such as power-driven motorcycle and power-driven assistance bicycle.
Further, in the said power field, pulse discharge characteristic of high current becomes important due to frequent on/off of the power supply, but enough pulse discharge capacity cannot be obtained by the high capacity-type non-aqueous electrolyte battery used in the mobile field and the like. On the other hand, high current discharge is possible by a high output-type power supply capacitor and the like, but long-term continuous discharge is difficult due to very small capacity.